1. Field of the Invention
The present invention relates to a runner vane for axial-flow hydraulic machinery and, more specifically, to techniques that improve its performance by optimizing the shape of a runner vane.
2. Description of the Related Art
Kaplan turbines and bulb turbines are known axial-flow hydraulic machines. FIG. 14 shows an essential part of a hydroelectric power station equipped with an axial-flow hydraulic turbine in a sectional view. Referring to FIG. 14, running water flows downstream into a casing 1. Then, the running water flows through stays 2 and movable guide vanes 3 for regulating the flow rate of the running water to a runner provided with runner vanes 4 and connected to a generator by a main shaft.
The running water exerts force on the runner vanes 4 to rotate a boss 5 holding the runner vanes 4. Consequently, the generator is driven to generate electric power. The running water flows through the runner vanes 4 into a draft tube 6. Then, the running water flows downstream or is discharged into a lower reservoir through the draft tube 6.
FIG. 15 shows the runner vane. Since a gap is formed between the tip 7 of the runner vane 4 and a discharge ring 8, the running water flows as a leakage flow through the gap. When the amount of the leakage flow is large, the hydraulic force of the running water cannot be fully imparted to the runner vanes 4 and the loss increases.
When the runner vanes 4 do not overlap each other in a section perpendicular to the axis “C” of the runner, the running water flows as a through flow in a circumferential space where the velocity of the running water is particularly high, namely, a space in the vicinity of the tips 7 of the runner vanes 4, without acting on the runner vanes 4.
Distribution of flow velocity is liable to be disturbed at the trailing edge of the runner vanes 4 in the vicinity of the tips 7 by the through flow of the running water and the gap between the runner vanes 4 and the discharge ring 8. The pressure of the disturbed flow cannot be satisfactorily recovered in the draft tube 6 resulting in performance deterioration.
Centrifugal force causes the running water to tend to flow toward the circumferential space in the vicinity of the tips 7 and the running water flowing at high velocity in the circumferential space causes the pressure on the suction surfaces, namely, the back surfaces, of the runner vanes 4 to drop.
Consequently, cavitation is liable to occurs in a part including the gap between the runner vanes 4 and the discharge ring 8 causing cavitation erosion. Thus, the suppression of cavitation is important as well as the reduction of loss that cause performance degradation for the extension of the runner vanes 4. Known techniques relevant to the present invention are disclosed in Japanese Patent documents 1, 2 and 3 as follows.
Patent document 1: JP-A H3-151570
Patent document 2: JP-A H7-54752
Patent Document 3: JP-A 2005-315216